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"A Stonehenge tunnel sounds completely fantastic to us!"
THE DAILY ASTRONOMER
Monday, January 16, 2017
Week 20 Night Sky Calendar
Imagine winter as a long tunnel burrowed through ice. Each year we trudge through this conduit and occasionally glance upward through the sheaths of arctic blue ice in desperate search for the remote Sun. We console ourselves with an assurance that every step we take through the tunnel conveys us all the closer to its exit. Today, as we pass January's mid point, we glimpse a hopeful future. For instance, let's contrast the solstice sun set time and the sun set time today. (This is only for Portland, Maine, so the times will be slightly different if you live in Boston, Caribou or Antananarivo, Madagascar.) On December 21st, Sol slumbered at 4:07 p.m. Civil twilight -the well-lit period immediately following sunset, often known simply as "dusk"- ended at 4:40 p.m.. On January 16, the Sun sets at 4:31 p.m. Civil twilight ends at 5:03 p.m. Now, on the summer solstice, the sun will set at 8:26 p.m and civil twilight will end at 9:02 p.m. We're certainly not at this point, yet, but with the Sun setting about 1 - 2 minutes later each day, we're moving in the right direction.
Our Winter Hexagon tour continues this week. Our next stop is Rigel in Orion. However, CJ, the Orion piece is a bit long winded, so I put it at the end of the Night Sky Calendar.
Most of this week's astronomical events this week are all clustered on Thursday! If the night sky would just allow humans to do the scheduling, astronomy would be so much improved.
THURSDAY, JANUARY 19 JUPITER 3 DEGREES SOUTH OF THE MOON
We should recall last week's sky watching tip. Jupiter will appear to be three degrees south of the moon around midnight. By the time Jupiter and the moon rise later in the evening, their separation distance will have increased to about 5 - 6 degrees. As they travel through the sky tonight, the separation distance will widen by about half a degree each hour. The last quarter moon, being fifty percent illuminated, is still bright, but won't obscure the brilliant planet Jupiter.
THURSDAY, JANUARY 19: MERCURY AT GREATEST ELONGATION (W 24 DEGREES)
Last week, Venus was at greatest eastern elongation, meaning it was visible in the western evening sky. This week, Mercury is at greatest western elongation, so it is visible in the eastern pre-dawn sky. Whereas Venus can stray as far as 47 degrees from the Sun, Mercury's greatest possible elongation cannot be more than 28 degrees. Today, it falls slightly short of its maximum, but it is still easier to observe now than it will be for the rest of this apparition
THURSDAY, JANUARY 19: LAST QUARTER MOON
SATURDAY, JANUARY 22: MOON AT APOGEE
It's simple.
Perigee - when the moon is closest to Earth
Apogee - when the moon is farthest from Earth
The moon's distance from Earth changes continually because its orbit isn't perfectly circular. Instead, its orbit is elliptical. We measure an ellipse's departure from circularity with eccentricity. An ellipse with an eccentricity of zero is a perfect circle. If its eccentricity is 1, it becomes a parabola. The moon's eccentricity is 0.054: almost a circle.
The moon's mean distance is about 239,000 miles. Its average perigee distance is 239,000(1 - 0.054) or 226,094 miles, while its average apogee distance would be 239,000(1 + 0.054) =251906 miles. These values vary somewhat due to the influences of other bodies, principally that of the Sun. Its apogee distance this time is 251,046 miles.
ECCENTRICITIES. This diagram, compliments of NASA.gov, shows how an ellipse's departure from circularity increases with increasing eccentricity. Whereas an ellipse with zero eccentricity is a perfect circle, an ellipse with an eccentricity of 0.9 is much more of a flattened oval. No planetary orbit in our solar system is nearly this elongated. Mercury's eccentricity is 0.206 and Pluto's is 0.248. No planetary orbit is perfectly circular, either. Venus's orbit is the least elongated, with an eccentricity of 0.007. Image: NASA.gov
PLANET WATCH
MERCURY: A beautiful morning planet this month, Mercury rises earlier each morning until late January. The little planet reaches its greatest western elongation on the 19th, when it will be 24 degrees west of the Sun. VERDICT: Mercury is never an easy planet to observe, but it is slightly easier this month if you venture out before sunrise and look to the east.
MERCURY: A beautiful morning planet this month, Mercury rises earlier each morning until late January. The little planet reaches its greatest western elongation on the 19th, when it will be 24 degrees west of the Sun. VERDICT: Mercury is never an easy planet to observe, but it is slightly easier this month if you venture out before sunrise and look to the east.
VENUS: (PICK PLANET!) Yes, we knew that you knew that we'd choose Venus as the pick planet again. How could we not? Our sister planet is not only at its greatest brilliance, but reached its greatest eastern elongation on January 12th. VERDICT: Unless you hate astronomy, you will want to see Venus this month. And, if you do hate astronomy, um, well, we hope you enjoy the DA, anyway.
MARS: Venus' evening sky companion, Mars is more than a hundred times dimmer, but still brighter than all but fifteen night sky stars. If one can find Venus, one need only look a bit to the west to find Mars. VERDICT: It doesn't dazzle, but Mars is still moderately bright and therefore not to difficult to observe.
JUPITER: A morning sky object this month. The giant planet rises well after midnight, but will be high in the pre-dawn eastern sky. Jupiter is that bright "star" one sees in the morning just before sunrise. VERDICT: A beautiful planet that one can even see as the morning twilight brightens.
SATURN: Yet another morning planet. However, Saturn remains low in the eastern pre-dawn sky throughout the month. It rises about four minutes earlier each day. As winter progresses, Saturn will move into greater morning sky prominence. We can look forward to seeing Saturn in the evening sky when the spring and summer warmth returns. VERDICT: You can find it if you absolutely have to. However, Saturn will be on stage almost all year and will be easier to find as we move into spring.
And, now, for Orion.
Being a bit of an Orion freak, I have written about this constellation many times before. To save time, I have re-posted a previous article about this grand and mighty hunter. (And, yes, I know it is just a collection of stars and that Orion is merely a figment produced by our collective tendency to impose human characteristics on celestial objects so as to elevate ourselves above ourselves, but, to be honest, being high above yourself is a beautiful state as the view is stunning and the endorphin flow never abates.)
Oh, and a brief piece about Rigel is tacked on at the end.
ORION
ORION THE HUNTER: Front and center around 9:00 p.m. in mid January, this giant hunter dominates the winter evening sky. Notice the four star rectangle enclosing Orion's belt. Rigel, one of the Winter Hexagon stars, occupies the lower right corner of this rectangle. Image by Matthew Spinelli
Orion has now appeared in our eastern early evening sky. His return doesn't occasion him the same ecstasy that so profoundly affects the rest of us. After all, Orion has been milling about the sky for more than a century of centuries: he loomed high in the firmament when the Stonehenge builders convened their first meetings on the English plain; looked upon the flames and revelry of Rome's Saturnalia festivals; was largely ignored by the Bayeux tapestry weavers; revered by Doctor Faustus; studied by Thomas Jefferson and observed by Harry Potter. And, he lingered about thousands of years before Stonehenge was even just a blue print just as he will be traipsing through the heavens centuries after Harry becomes a figure of ancient literature. To Orion, this most recent return is just the latest go around in an interminable circuit through Earth's skies. (As Orion's stars are quite distant, he is likely poised high above many nearby alien planet scapes, as well.)
Tonight, Orion will have completely risen by 6:00 p.m. One can find the grand hunter on its side, as though in repose, just above the eastern early evening horizon.
A large rectangle enclosing a diagonal three star belt comprises his main pattern. Two star lines curling out from his northeastern rectangle star, Betelgeuse, form a bronze club. A semi circle of stars surrounding the northwestern rectangle star, Bellatrix, represents a lion skin shield. A faint sword dangles from his belt. A solitary star marks his head.
Mythologically, Orion has assumed various identities: the Egyptian Death God Osiris; the grand Hebrew hunter Nimrod; the Sioux warrior Long Sash; and in some traditions was Feryja, a Nordic Goddess. One will find other Orion incarnations in various African, Asian and other cultural folklore around the world. Since the celestial equator -the projection of Earth's equator onto the sky- passes just above the belt, Orion is wholly visible to more than 99% of the world's population. (Only the chest and shoulder section appear around the North Pole; whereas a South Pole observer sees only the belt and legs.) His widespread visibility. his stark resemblance to a standing human, and his collection of brilliant stars have assured Orion a global fame surpassing that of any other single star pattern.
Of all his other identities, he is now principally known as Orion, considered the greatest hunter in Greco-Roman mythology, superior even to Artemis, Goddess of the hunt and the Moon. One tale portrays Orion as reckless and boastful. So proud was he of his hunting prowess that he openly proclaimed himself capable of slaying any beast, no matter how formidable. Appalled by Orion's impertinent attitude -such irrational arrogance was only deemed proper in Olympus- Artemis deployed Scorpius the Scorpion onto Earth. The Scorpion promptly found Orion and nipped his ankle, killing him instantly. Curiously, Artemis then foisted Orion's spirit into the sky and bejeweled him with bright stars as a tribute to his superb hunting skills. He was perhaps also set into the stars as a warning to other gifted mortals that it is best for one to remain quiet about one's exceptional abilities.
And there he remains on full display as he has for such a long time. We should point out that even Orion is ultimately mortal. His component stars all move through the galaxy in various directions and after tens of thousands of years, the pattern will become noticeably distorted and ultimately unrecognizable. For us mortals, Orion's life is interminable. However, by galactic time periods, Orionis a fleeting configuration: just one of the myriad star patterns through the galaxy that form and dissolve in deep time.
RIGEL An artistic impression of the blue supergiant star marking Orion's western knee. This hot star is nearly half a million times larger (in volume) and 40,000 times brighter than the Sun.
Even though Betelgeuse is Orion's most famous star, Rigel is the constellation's brightest. Representing the western knee, Rigel is a blue supergiant nearly 500,000 times larger than the Sun in terms of volume. With an effective temperature of more than 12,000 degrees C, Rigel produces more than 40,000 times the amount of energy the Sun releases. Rigel is ultra-energetic, as it is not only hot, but highly massive: more than 23 times more massive than the Sun. Being only about eight million years old, it is also young. Despite its youth, Rigel has exhausted its core hydrogen reserves and has expanded to the supergiant stage in the process. Eventually -in a few million years- Rigel will finally reach the end-stage and explode as a supernova. When this cataclysmic explosion occurs, Rigel will become the brightest object in our night sky apart from the moon. Currently, it is a highly massive and furiously energetic star that burns blue-hot in our winter night sky